Implantable cardioverter defibrillators (ICDs) typically have the capability of delivering both low voltage therapies and high voltage therapies in response to monitoring a cardiac rhythm and detecting a need for therapy. Low voltage therapies may include bradycardia pacing, cardiac resynchronization therapy (CRT), and anti-tachycardia pacing (ATP). Low voltage therapies are typically delivered using low voltage pacing electrodes, e.g. tip or ring electrodes delivering pulses of 5 Volts or less in amplitude.
High voltage therapies such as cardioversion or defibrillation shocks are delivered in response to detecting ventricular tachycardia or ventricular fibrillation. High voltage therapies are typically delivered using high voltage coil electrodes and the housing of the ICD, often referred to as the “CAN electrode” or a “housing electrode.” High voltage electrodes generally have a greater surface area and deliver high energy shock pulses, typically in the range of at least 10 Joules and up to 35 Joules.
A single lead coupled to an ICD may carry multiple electrodes, which may include either or both high voltage and low voltage electrodes. Each electrode is coupled to an electrically insulated conductor extending through the elongated lead body to facilitate electrical connection of each therapy delivery electrode to the ICD.
Short circuit conditions can sometimes occur when a therapy delivery electrode or its conductor makes electrical contact with another conductor or electrode. Lead integrity testing may be performed regularly to make lead measurements, such as lead impedance measurements, to monitor for possible short circuit, open circuit or other lead conditions. A non-contact high voltage lead fault can exist, however, and may manifest only when a high-voltage therapy is delivered, causing arcing between exposed conductors. These types of faults involving high voltage conductors are frequently undetected by low voltage lead measurements. A high voltage short circuit that occurs during delivery of a defibrillation shock is likely to prevent adequate energy from being delivered to the heart, leading to a failed therapy. Since ventricular fibrillation is a life-threatening condition, prompt detection of a potential high voltage short circuit condition is needed.